What is the role of somatosensory evoked potentials (SEPs) in intraoperative monitoring?

Updated: Feb 26, 2019
  • Author: Sombat Muengtaweepongsa, MD, MSc; Chief Editor: Selim R Benbadis, MD  more...
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A comprehensive discussion of the interpretation of intraoperative evoked potential data is beyond the scope of this presentation and the reader is referred to other sources. When surgical maneuvers compromise neural tissue, SEP components may show significant amplitude attenuation before their latencies become prolonged. Thus, both amplitudes and latencies should be evaluated during intraoperative monitoring. No universally accepted standard exists for what constitutes a significant change, but a 50% decrease in the amplitude of an SEP component or a 10% increase in its latency often are used as alarm threshold criteria. The differences in the percentages reflects the fact that SEP amplitudes generally show more run-to-run variability than do SEP latencies.

Whenever the SEPs change, the interpreter must distinguish between the many possible causes of such a change, which include anesthetic effects and technical factors as well as true neuronal damage or dysfunction. As discussed above, recordings of peripheral nerve CAPs and cervicomedullary far-field potentials can help elucidate the causes and significance of changes in cortical SEPs during intraoperative SEP monitoring for both upper limb and lower limb SEPs.

Anesthetic agents are probably the most common cause of intraoperative SEP changes. In general, the longer the latency of an SEP component and the more synapses between the stimulation site and the component's neural generator, the greater is the degree to which that component will be affected by anesthetic agents. Thus, anesthetic effects may alter the cortical SEPs while sparing the far-field SEPs (see images below), mimicking surgery-related dysfunction of the cerebral cortex or of the pathways from the brain stem to the cerebral cortex. Personnel performing intraoperative monitoring must pay careful attention to the anesthetic regimen and should record it periodically on their data logs.

Cortical (left) and cervicomedullary N14 (right) s Cortical (left) and cervicomedullary N14 (right) somatosensory evoked potentials (SEPs) to stimulation of the right median nerve, recorded during the initial phases of surgery for resection of a right vestibular schwannoma. The cortical SEPs show prominent anesthetic-related changes. While the waveforms recorded in the A2-Fpz channel contain some volume-conducted cortical SEPs, the N14 far-field component (arrowhead) is unaffected by the changes in the anesthetic regimen. Courtesy of Legatt, 1995.
Serial somatosensory evoked potentials (SEPs) reco Serial somatosensory evoked potentials (SEPs) recorded during spinal instrumentation and fusion surgery in a 13-year-old girl with scoliosis. Note the attenuation of the cortical SEPs resulting from administration of an intravenous bolus dose of 50 mg of fentanyl given at 1:53 pm. The far-field SEPs were relatively unaffected. In addition to the far-field components, the C2S-Fpz waveforms (labeled "SC2-Fpz") contain a volume-conducted contribution from the cortical SEPs; the contribution also was attenuated by the fentanyl. Nitrous oxide (60%) and isoflurane (0.6-0.8%) were being administered throughout these recordings. Positivity at input 1 is shown as an upward deflection in this picture.

Personnel performing intraoperative monitoring also should periodically note and log the temperature and blood pressure of the patient, which also can affect the electrophysiologic signals. Anesthetic-induced changes typically are bilateral; this can help distinguish anesthetic-related from surgery-related SEP changes when the latter are expected to be unilateral (eg, during carotid endarterectomy) but not when surgical manipulations can damage afferent sensory pathways bilaterally (eg, bilateral spinal cord damage during surgery for scoliosis). Peripheral and rostral or contralateral cortical SEPs should help to exclude non-iatrogenic etiology. [14] Area under the curve (AUC) of the SEPs may help to promtly detect intra-operative spinal cord damage. [15]

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